| Year | Value | | 2022 | 48.928097554375 | | 2021 | 48.904077270499 | | 2020 | 48.893865805544 | | 2019 | 48.88376620202 | | 2018 | 48.873794271967 | | 2017 | 48.862863055712 | | 2016 | 48.850982802156 | | 2015 | 48.837364933242 | | 2014 | 48.821929273907 | | 2013 | 48.805130802847 | | 2012 | 48.785320635547 | | 2011 | 48.762282810296 | | 2010 | 48.736477459039 | | 2009 | 48.708208846359 | | 2008 | 48.676649851454 | | 2007 | 48.643035636787 | | 2006 | 48.606894594978 | | 2005 | 48.567005052996 | | 2004 | 48.524857721208 | | 2003 | 48.480538402831 | | 2002 | 48.43286893247 | | 2001 | 48.3829289615 | | 2000 | 48.330379693845 | | 1999 | 48.301047875982 | | 1998 | 48.288720920261 | | 1997 | 48.267724467951 | | 1996 | 48.242671646778 | | 1995 | 48.213833561219 | | 1994 | 48.181506426524 | | 1993 | 48.144800980553 | | 1992 | 48.102152851278 | | 1991 | 48.053013606746 | | 1990 | 47.995965092497 | | 1989 | 47.932032044761 | | 1988 | 47.861908944326 | | 1987 | 47.780189023648 | | 1986 | 47.698471697257 | | 1985 | 47.637087870457 | | 1984 | 47.589702627345 | | 1983 | 47.540597424163 | | 1982 | 47.486513486514 | | 1981 | 47.430601214918 | | 1980 | 47.371056370577 | | 1979 | 47.306298884785 | | 1978 | 47.231121071712 | | 1977 | 47.139836534997 | | 1976 | 47.021785360786 | | 1975 | 46.90350917095 | | 1974 | 46.820535505191 | | 1973 | 46.759913497773 | | 1972 | 46.714794695287 | | 1971 | 46.682349490026 | | 1970 | 46.72082299098 | | 1969 | 46.793627040541 | | 1968 | 46.811365285748 | | 1967 | 46.81117575955 | | 1966 | 46.809290481886 | | 1965 | 46.804845044391 | | 1964 | 46.798636939482 | | 1963 | 46.794519786185 | | 1962 | 46.792961341566 | | 1961 | 46.789004240386 | | 1960 | 46.788818944844 |
|